UNDERSTANDING THE RESULTS

1. What were the basic findings of this study?

Antibiotic use is associated with an increased risk of breast cancer. The authors found that the more antibiotics the women in the study had used, the higher their risk of breast cancer.

2. What do these results mean?

These study results do not mean that antibiotics cause cancer. These results only show that there is an association between the two. More research is necessary to learn if there is a direct cause-and-effect relationship.

3. What specifically did the authors find?

The authors found that women who took antibiotics for more than 500 days—or had more than 25 prescriptions—over an average period of 17 years had twice the risk of breast cancer as women who had taken no antibiotics. The risk was smaller for women who took fewer antibiotics, but even women who had between one and 25 prescriptions over an average period of 17 years had an increased risk; they were about 1.5 times more likely to be diagnosed with breast cancer than those women who didn't take any antibiotics. The authors found the increase in risk across all classes of antibiotics that they studied.

4. Should women not take antibiotics, even if their doctor tells them that they have a bacterial infection?

No. There is no reason to avoid antibiotics for bacterial infections; antibiotics can have a significant health benefit for such infections. However, this study provides more reason for people to use antibiotics wisely—to take antibiotics only for bacterial infections and not to use them to try to treat common colds and other illnesses caused by viruses.

5. What should women do who take antibiotics long-term for conditions such as acne or recurrent bladder infections?

Women should talk to their healthcare providers about the risks and benefits of antibiotic use for their condition(s). Antibiotics should be used as recommended only when patients and their physicians have determined that they are the best treatment. In addition, women should follow their doctor's recommendations regarding breast cancer screening.

6. Is it possible that other risk factors for breast cancer could explain the increased risk seen in this study?

Factors such as age, reproductive and menstrual history, lifestyle factors, use of hormone therapy, and high breast density, among other factors, can affect breast cancer risk. The authors designed the JAMA study in such a way as to minimize the chance that these and other factors would affect the results. They matched the “cases”—women who were diagnosed with breast cancer—and “controls”—women who did not have breast cancer—with regard to these factors. For example, 23.7 percent of cases had a body mass index of more than 30.0 kg/m2 and almost the same number (21.8 percent) of controls had the same body mass index, lowering the chance that the higher risk of breast cancer seen in the case group was due to obesity.

Additional information about breast cancer and risk factors is available at http://www.cancer.gov/cancertopics/types/breast or by calling the NCI's Cancer Information Service at 1–800–4–CANCER (1–800–422–6237).

7. What factors, other than antibiotic use, might explain why the women in this study have an increased risk of breast cancer?

It is possible that the conditions that required the women to take the antibiotics might have increased their risk of breast cancer. For example, a weakened immune system might lead both to infections that require antibiotics and to cancer development.

It is also possible that women who have not taken any antibiotics may be a uniquely healthy group in terms of general well-being or lifestyle factors, possibly accounting for some of the increased breast cancer risk that the authors observed among antibiotic users.

8. Alternatively, if there is a cause-and-effect relationship, how do researchers think that antibiotic use might lead to breast cancer?

Based on current understanding, there are a couple of possible explanations for why antibiotics might lead to breast cancer. One theory is that antibiotics can affect bacteria in the intestine, which may impact the way certain foods that protect against cancer are broken down in the body. Another possibility is that antibiotics can affect the body's immune response and response to inflammation, both of which could be related to the development of cancer. Many more studies are necessary to better understand how or why antibiotics could lead to cancer.

ABOUT THE STUDY

9. Why did the authors conduct this study?

Breast cancer is the second leading cause of cancer deaths among women in the United States. It is estimated that 40,110 women will die of the disease in 2004. In addition, breast cancer is the most common cancer in women worldwide.

Antibiotics are used—and overused—extensively throughout the world to treat many conditions, illnesses, and diseases. For example, more than 70 percent of women in the study had used between one and 25 prescriptions for antibiotics in the past 17 years. Only 18 percent of women in the study had not filled any antibiotic prescriptions during their enrollment in the health plan. About 2 percent to 3 percent of the women in the study had used more than 50 prescriptions over an average time period of 17 years. For more information about antibiotics, please see Questions 12 and 13.

The idea that antibiotics might increase cancer risk was first proposed decades ago. Before this study in the Journal of the American Medical Association, there had been only one study to examine the association between antibiotics and cancer risk, and it also focused on breast cancer. That study, of almost 10,000 women in Finland, was less thorough in its evaluation of antibiotic use, but still found that increased antibiotic use was associated with an increased risk of breast cancer.

10. How was this JAMA study conducted?

The authors used computerized pharmacy and breast cancer screening databases at Group Health Cooperative in Seattle, a large, non-profit health plan in Washington state. They compared the antibiotic use of 2,266 women with breast cancer to similar information from 7,953 women without breast cancer. All the women in the study were age 20 and older, and the researchers examined a wide variety of the most frequently prescribed antibiotic medications.

11. Who funded this study?

The study was funded by the National Cancer Institute and by the Gustavus and Louise Pfeiffer Research Foundation.

ABOUT ANTIBIOTICS

12. What conditions are antibiotics used to treat?

Antibiotics are used to fight bacterial infections. They are commonly prescribed to treat bladder and respiratory infections, acne, and rosacea, among other conditions. Rosacea is a disease that affects the skin and sometimes the eyes, causing redness, pimples and, in advanced stages, thickened skin.

13. What should people know about antibiotics?

Antibiotics can show substantial results when used to treat bacterial infections and should be taken exactly as prescribed by a healthcare provider. It is important to note, however, that antibiotics won't help at all against the flu or the common cold, which are caused by viruses, not bacteria. Over the past decade, overuse of antibiotics has become a serious problem. According to the Centers for Disease Control and Prevention (CDC), tens of millions of antibiotics are prescribed for viral infections that are not treatable with antibiotics, contributing to the alarming growth of antibiotic resistance.

To address this growing health concern, efforts are underway such as the “Get Smart: Know When Antibiotics Work” campaign—unveiled last year by the Department of Health and Human Services’ CDC and the Food and Drug Administration (FDA) and other partners—to lower the rate of antibiotic overuse. For more information about the “Get Smart: Know When Antibiotics Work” campaign, please visit http://www.cdc.gov/drugresistance/community.

FUTURE WORK

14. What studies must be done to understand more about the link between antibiotic use and breast cancer?

Scientists must conduct additional studies to determine what causes the increased risk of breast cancer with antibiotic use. In addition, more studies are necessary to determine whether the risk of breast cancer is higher for women who used antibiotics at a certain period of their life, such as during adolescence or during pregnancy, or whether women who used antibiotics for specific conditions have a higher risk than women who used the drugs to treat other conditions.

Resources

To view the NCI press release on this JAMA study go to http://www.cancer.gov/newscenter/pressreleases/antibiotics.

Additional information about breast cancer is available at http://www.cancer.gov/cancertopics/types/breast.

For information about cancer, please visit the NCI home page at http://www.cancer.gov or call the NCI's Cancer Information Service at 1–800–4–CANCER (1–800–422–6237).

# # #

• Cancer Causes and Risk Factors Home Page ¹
  (http://www.cancer.gov/cancertopics/prevention-genetics-causes/causes)
• What You Need To Know About™ Breast Cancer ²
  (http://www.cancer.gov/cancertopics/wyntk/breast)

How can we help?

We offer comprehensive research-based information for patients and their families, health professionals, cancer researchers, advocates, and the public.

• Call NCI’s Cancer Information Service at 1–800–4–CANCER (1–800–422–6237)
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Glossary Terms

A disorder of the skin in which oil glands and hair glands become inflamed.

A drug used to treat infections caused by bacteria and other microorganisms.

A large group of single-cell microorganisms. Some cause infections and disease in animals and humans. The singular of bacteria is bacterium.

The organ that stores urine.

A measure that relates body weight to height. BMI is sometimes used to measure total body fat and whether a person is a healthy weight. Excess body fat is linked to an increased risk of some diseases including heart disease and some cancers. Also called BMI.

Cancer that forms in tissues of the breast, usually the ducts (tubes that carry milk to the nipple) and lobules (glands that make milk). It occurs in both men and women, although male breast cancer is rare.

Describes the relative amount of different tissues present in the breast. A dense breast has less fat than glandular and connective tissue. Mammogram films of breasts with higher density are harder to read and interpret than those of less dense breasts.

A term for diseases in which abnormal cells divide without control and can invade nearby tissues. Cancer cells can also spread to other parts of the body through the blood and lymph systems. There are several main types of cancer. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Sarcoma is a cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue. Leukemia is a cancer that starts in blood-forming tissue such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the blood. Lymphoma and multiple myeloma are cancers that begin in the cells of the immune system. Central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord. Also called malignancy.

The process of identifying a disease, such as cancer, from its signs and symptoms.

Any substance, other than food, that is used to prevent, diagnose, treat or relieve symptoms of a disease or abnormal condition. Also refers to a substance that alters mood or body function, or that can be habit-forming or addictive, especially a narcotic.

An agency in the U.S. federal government whose mission is to protect public health by making sure that food, cosmetics, and nutritional supplements are safe to use and truthfully labeled. The FDA also makes sure that drugs, medical devices, and equipment are safe and effective, and that blood for transfusions and transplant tissue are safe. Also called Food and Drug Administration.

An agency in the U.S. federal government whose mission is to protect public health by making sure that food, cosmetics, and nutritional supplements are safe to use and truthfully labeled. The Food and Drug Administration also makes sure that drugs, medical devices, and equipment are safe and effective, and that blood for transfusions and transplant tissue are safe. Also called FDA.

Treatment that adds, blocks, or removes hormones. For certain conditions (such as diabetes or menopause), hormones are given to adjust low hormone levels. To slow or stop the growth of certain cancers (such as prostate and breast cancer), synthetic hormones or other drugs may be given to block the body’s natural hormones. Sometimes surgery is needed to remove the gland that makes a certain hormone. Also called endocrine therapy, hormonal therapy, and hormone treatment.

The activity of the immune system against foreign substances (antigens).

The complex group of organs and cells that defends the body against infections and other diseases.

Invasion and multiplication of germs in the body. Infections can occur in any part of the body and can spread throughout the body. The germs may be bacteria, viruses, yeast, or fungi. They can cause a fever and other problems, depending on where the infection occurs. When the body’s natural defense system is strong, it can often fight the germs and prevent infection. Some cancer treatments can weaken the natural defense system.

Redness, swelling, pain, and/or a feeling of heat in an area of the body. This is a protective reaction to injury, disease, or irritation of the tissues.

The long, tube-shaped organ in the abdomen that completes the process of digestion. The intestine has two parts, the small intestine and the large intestine. Also called bowel.

A measure of weight. A kilogram is equal to 2.2 pounds.

The monthly cycle of hormonal changes from the beginning of one menstrual period to the beginning of the next.

The National Cancer Institute, part of the National Institutes of Health of the United States Department of Health and Human Services, is the Federal Government's principal agency for cancer research. The National Cancer Institute conducts, coordinates, and funds cancer research, training, health information dissemination, and other programs with respect to the cause, diagnosis, prevention, and treatment of cancer. Access the National Cancer Institute Web site at http://www.cancer.gov. Also called NCI.

A condition marked by an abnormally high, unhealthy amount of body fat.

Medical doctor.

A doctor's order for medicine or another intervention.

The organs involved in producing offspring. In women, this system includes the ovaries, the fallopian tubes, the uterus, the cervix, and the vagina. In men, it includes the prostate, the testes, and the penis.

Something that increases the chance of developing a disease. Some examples of risk factors for cancer are age, a family history of certain cancers, use of tobacco products, being exposed to radiation or certain chemicals, infection with certain viruses or bacteria, and certain genetic changes.

A person who has studied science, especially one who is active in a particular field of investigation.

Checking for disease when there are no symptoms. Since screening may find diseases at an early stage, there may be a better chance of curing the disease. Examples of cancer screening tests are the mammogram (breast), colonoscopy (colon), Pap smear (cervix), and PSA blood level and digital rectal exam (prostate). Screening can also include checking for a person’s risk of developing an inherited disease by doing a genetic test.

In medicine, a very simple microorganism that infects cells and may cause disease. Because viruses can multiply only inside infected cells, they are not considered to be alive.